Elision / Return Value Optimization (RVO)

In certain cases, compiler can skip calling copy / more constructers, instead writing the object’s value s directly into its final location.

Consider the example below:

Rational makeRational() {return Rational{1, 5}};
Rational r = makeRational();

• In g++ std=c++14, neither move nor copy gets called. Elision is used

Another example

void doMath(Rational r) {
	...
}
doMath(makeRational());

Here, {1,5} is directly written into r.

Note: Elision is possible, even if it changes program output.

• Not expected to know all possible cases, just that it’s possible.

Disable with the flag -fno-elide-constructors (slows down program though)

Copy Elision

https://en.wikipedia.org/wiki/Copy_elision https://stackoverflow.com/questions/12953127/what-are-copy-elision-and-return-value-optimization

In the context of the C++ programming language, Return Value Optimization (RVO) is a compiler optimization that involves eliminating the temporary object created to hold a function’s return value.